1. Field of the Invention
The invention relates to a process for preparing silanes which have an isocyanate function, in which improved storage stability is achieved.
2. Description of the Related Art
There has for a long time been great interest in an economical method of preparing isocyanatoorganosilanes in high yields and purities. The compounds mentioned are of great economic importance.
Isocyanatosilanes can be used, for example, as bonding agents between organic polymers and inorganic materials. However, isocyanatoorganosilanes are used in industry first and foremost for the termination of organic polyols (e.g. polyether polyols, polyurethanes, polyesters or poly(meth)acrylates. The resulting alkoxysilane-terminated prepolymers cure on contact with atmospheric moisture and are used, inter alia, as adhesives and sealants or as surface coating resins or as constituents of surface coating resins.
In the prior art, use is usually made of conventional γ-isocyanatopropylsilanes of the formula (1)OCN—(CH2)3—Si(OR′)x(R″)3-x  (1),where R′ and R″ are alkyl radicals and x is 0-3, preferably 3 or 2.
However, there has recently been particular interest in the α-isocyanatomethylsilanes of the general formula (2)OCN—CH2—Si(OR′)x(R″)3-x  (2),where R′ and R″ and x are as defined above.
These α-isocyanatomethylsilanes have a particularly high reactivity toward atmospheric moisture and are suitable for preparing alkoxysilane-terminated prepolymers having a high but regulatable curing rate (described, for example, in EP 1 421 129 and WO 2004/022618). In addition, the corresponding α-silane-terminated prepolymers can also be crosslinked without the tin catalysts which are of toxicological concern (described, inter alia, in EP 1 421 129).
Various processes for preparing isocyanatoorganosilanes are known. Thus, EP 0 212 058 describes a process in which silanes having a urea unit are thermally dissociated in the liquid phase to form isocyanatoorganosilanes and the corresponding amines or amides.
EP 1 010 704 discloses a process for preparing isocyanatoorganosilanes, in which carbamatoorganosilanes are thermally dissociated in the liquid phase in the presence of tin(II) chloride as catalyst to form the corresponding isocyanatoorganosilanes.
DE 101 08 543 describes the preparation of isocyanatoorganosilanes from the corresponding carbamatoorganosilanes and alkylchlorosilanes or vinylchlorosilanes.
Furthermore, U.S. Pat. No. 6,008,396 discloses a process in which carbamatoorganosilanes are converted in inert hot media with elimination of alcohol into the corresponding isocyanatosilanes which are then removed directly from the reaction mixture by distillation.
The thermal dissociation of carbamatoorganosilanes to form isocyanatoorganosilanes and methanol is also described in EP 0 649 850 where the dissociation takes place in the gas phase under atmospheric or reduced pressure. The reaction is preferably carried out in a tube reactor. An improvement in this process, in which the carbamate dissociation is carried out in the presence of a heterogeneous catalyst is also known from EP 1 343 793. If appropriate, the thermolysis can also be carried out using the microstructure apparatuses described in WO 2005/056565.
A further process in which the isocyanatoorganosilanes are prepared under the action of microwaves is described in WO 2005/056564. Finally, WO 2005/055974 describes this microwave process in combination with fluidizing solid particles.
The processes described in the prior art are usually used only for preparing the conventional γ-isocyanatopropylsilanes of the formula (1). However, they are, at least in principle, also suitable for preparing α-isocyanatomethylsilanes of the formula (2). Without exception, though, they have the disadvantage that the α-isocyanatomethylsilanes of the formula (2) obtained by all the processes are relatively unstable and, at least at room temperature, do not have a satisfactory storage stability. Thus, storage at room temperature for only a few weeks, sometimes even only a few days depending on the production process, is enough for decomposition of significant proportions (i.e. >>10%) of the respective α-isocyanatomethylsilane to be observed. It is also notable, in particular, that the decomposition of the α-isocyanatomethylsilanes also takes place in sealed containers with exclusion of air. Here, the α-isocyanatomethylsilanes of the formula (2) differ significantly from conventional isocyanates without a silane unit since the latter can usually be stored without problems for months in airtight containers.